present invention relates to sonar systems, and more particularly relates to a sonar transducer adapted to provide velocity measurements of a vessel moving in a body of water, or any motion of the water relative to the transducer.
Both surface and underwater vessels must have an accurate indication of their velocity and their position in the water to perform their intended missions. In some cases, position can be determined using radio and satellite navigational references. In other situations, such navigational references are not available or unusable. Thus, the vessel needs a navigation mechanism which does not rely on such surface based devices to determine its position and velocity. This need becomes more significant as the complexity or duration of the vessel's activities increase.
Likewise, accurate charting of currents is essential for safe navigation. A sonar transducer capable of tracking motion of water relative to the transducer will enable detailed profiling of water currents. Even more desirable is a sonar that could simultaneously track vessel motion through the water relative to the ocean floor, as well as track the water currents relative to the motion of the vessel.
In the past, vessel mounted sonar systems have existed in which the ocean bottom is used to reflect acoustic energy back to the ship. By measuring the Doppler shift such previously available systems attempted to accurately determine the vessel's speed over the bottom of the body of water. Such previously available schemes, sometimes called Doppler Speed Logs or Doppler velocity logs, have been utilized for navigation in docking of large ships for some time. Their objective is to measure a ships velocity with respect to the floor of the body of water and without reference to any land-based objects and without being influenced by wind and other adverse weather conditions.
In general, previously available Doppler Speed Log sonar systems have included sonar transducers to send and receive the acoustic energy. These transducers are hull mounted similar to echo sounder transducers. Preamplifiers are generally needed in the vicinity of the receiving transducer to amplify the weak return signal. Electronic circuitry is required to process the returned signal so that the frequency shift can be determined and a velocity may be computed. A method of display is also required to convert the electrical signal to a visual indication that can be used by the ship's crew.
In order for the motion of a ship to cause a frequency shift in a sonar transmission, the sonar beam must have a directional vector aligned with the motion of the ship. Disadvantageously, with many of the previously available devices relatively small trim changes of the ship can cause large apparent velocity changes. To eliminate this sensitivity to trim, an arrangement referred to as the Janus configuration is used. According to this configuration, beams are generated in the plus and minus thirty degree directions relative to a vertical primary axis. The beams extend fore and aft relative to the longitudinal axis of the ship.
Previously available Doppler Speed Log sonar systems have typically been of the pulse type since continuous wave systems cease to operate when the depth exceeds a predetermined amount, for example, 200 feet. This is because as the water becomes deeper, the number of scattering particles, such as air bubbles, increases the scattered signal begins to dominate over the signal reflected from the bottom.
In one previously available pulse Doppler Speed Log sonar system, commercially manufactured by the Marquardt Company, a two-axis transducer is utilized. This transducer has two separate sending and receiving faces, each aligned at an angle relative to the primary axis for generating the beams in the plus and minus thirty degree directions. The transducer is mounted in a housing on the bottom of the ship which creates a cavity where flow is disturbed and air bubbles can collect and seriously degrade the accuracy of the system.
Another previously available Doppler Speed Log sonar system referred to as Atlas-Dolog 10 has been commercially available from Krupp GMBH of Bremen, Germany. In that system, separate transmitter and receiver transducers are utilized. Each consists of a large number (72) of lead-zircon-titanate crystals. Each of the crystals has a flat, cylindrical shape. The crystals are embedded in a block of synthetic material. Complex electronic circuitry including drivers and phase shifters is utilized to generate the sonar beams.
A more recent prior art Doppler Speed Log incorporates a phased array Doppler sonar transducer. See U.S. Pat. No. 4,641,291 which is hereby incorporated by reference. The phased array includes a plurality of piezoelectric rectangular planar staves held in side-by-side relation in a laminate assembly by insulative spacers. The polarity of adjacent pairs of the staves is inverted relative to each other. This polarity is achieved by placing staves with positive sides "up" or "down". The acoustic centers of the staves are spaced apart a distance of approximately one-half wavelength of the operation frequency. The array of staves defines an active planar acoustic face for sending a pair of sonar beams which are angularly separated by about 60 degrees without electronically phasing or time delaying the signals transmitted to the individual staves and without mechanically rotating the array. The individual receive signals are obtained by a simple phase delay circuitry.
While the phased array Doppler sonar transducer described above is an advance over the previous state of the art, its fabrication is very labor intensive. Each stave must be properly oriented and must be individually placed in the array. Additionally, the staves must be placed precisely so that the acoustic centers of the staves are spaced apart one-half wavelength of the operating frequency. Furthermore, each stave has to be hand wired so that it will transmit and receive properly.
Thus, it would be an advance in the art to provide a transducer for use in a phased array Doppler sonar system whose fabrication is not labor intensive and which assures that the acoustic transducer components will be properly and precisely placed.